## 一、如何使用Collections工具类进行排序
使用Collections工具类进行排序主要有两种方式：
### 1.对象实现Comparable接口，重写compareTo方法
```java
/**
* @author Hanstrovsky
*/
@Data
@AllArgsConstructor
public class Student implements Comparable {
String name;
int age;
@Override
public int compareTo(Object o) {
Student stu = (Student) o;
//this-参数：升序；参数-this：降序
return this.age - stu.age;
}
}
```
```java
/**
* @author Hanstrovsky
*/
public class Test01 {
public static void main(String[] args) {
List stuList = new ArrayList<>();
stuList.add(new Student("abc", 17));
stuList.add(new Student("cab", 18));
Collections.sort(stuList);
System.out.println(stuList);
}
}
```
### 2.传入一个比较器对象Comparator。
还是用上面的student，不去实现Comparable接口。
```java
/**
* @author Hanstrovsky
*/
public class Test02 {
public static void main(String[] args) {
List stuList = new ArrayList<>();
stuList.add(new Student("abc", 19));
stuList.add(new Student("cab", 18));
Collections.sort(stuList, new Comparator() {
@Override
public int compare(Student o1, Student o2) {
return o1.age - o2.age;
}
});
System.out.println(stuList);
}
}
```
## 二、依据多个字段排序
当需求要求先按第一个字段排序，如果第一个字段相同，则按第二个字段排序，如果第二个相同，则按第三个字段... 可以定义多个Comparator,并依次使用。
```java
/**
* @author Hanstrovsky
*/
@Data
@AllArgsConstructor
public class Student {
// 编号
private String id;
// 身高
private int height;
// 体重
private int weight;
}
```
```java
/**
* @author Hanstrovsky
*/
public class Test03 {
public static void main(String[] args) {
Student s1 = new Student("1", 180, 80);
Student s2 = new Student("2", 175, 80);
Student s3 = new Student("3", 175, 90);
List students = new ArrayList<>();
students.add(s1);
students.add(s2);
students.add(s3);
System.out.println("原始排序：" + students);
//按照身高升序排序
Comparator byHeight = Comparator.comparing(Student::getHeight);
//按照体重升序排序
Comparator byWeight = Comparator.comparing(Student::getWeight);
//将list先按照"身高"升序再按照"体重"升序排列
students.sort(byHeight.thenComparing(byWeight));
//将list先按照"身高"升序再按照"体重"升序排列
System.out.println("优先身高：" + students);
//将list先按照"体重"升序再按照"身高"升序排列
students.sort(byWeight.thenComparing(byHeight));
//将list先按照"身高"升序再按照"体重"升序排列
System.out.println("优先体重：" + students);
}
}
```
